The present invention relates to a method for controlling the flow of fluids through subterranean formations.
It is well-known that subterranean formations comprise layers or zones of different permeabilities. In the recovery of hydrocarbon material such as oil or natural gas from a subterranean reservoir, highly porous or permeable zones often create significant problems.
For example, in enhanced or secondary oil recovery operations such as water or surfactant flooding wherein an aqueous fluid is injected into the formation to drive the hydrocarbon to a producing wellbore, a disproportionately high amount of the injected drive fluid bypasses through zones of high permeability into the producing wellbore without sweeping appreciable amounts of hydrocarbon from the reservoir. This greatly reduces the efficiency of the operation. In addition, excessive amounts of water are recovered along with the hydrocarbon.
Alternatively, water, normally in the form of brine, is commonly native to the formation. The communication of a water-containing strata with a producing wellbore via a highly permeable zone can cause excessive water to be produced along with the hydrocarbon. This results in a high pumping cost and a disposal problem for the recovered water.
Various solutions have been proposed heretofore to control the permeability of subterranean formations. For example, it has been proposed to place a solid plug of a material such as cement within the more permeable zones of the formation. Similarly, suspensions of finely divided solids have been pumped into the formation in an attempt to plug highly permeable zones. Materials employed in such attempts have included organic matter such as ground leather or ground walnut shells and inorganic materials such as clays and finely ground silica. Unfortunately, the use of solid plugs or finely divided solid powders has frequently proved unsuccessful due to the fact that the material fails to plug the zones of high permeability or, conversely, the material indiscriminately and permanently plugs both hydrocarbon bearing zones as well as other zones in the formation, thereby resulting in a permanent loss of the desired hydrocarbon fluid. Moreover, even when the operation is successful, completely plugging portions of the formation does not significantly increase the recovery operation.
Alternatively, it has been proposed to control the permeability of a formation and hence modify the mobility of a fluid through the formation using a viscous fluid pumped into the hydrocarbon-bearing formation. For example, U.S. Pat. No. 3,039,529 describes incorporating a partially hydrolyzed polyacrylamide into an aqueous drive fluid to increase its viscosity and hence to control its mobility through the formation. Unfortunately, substantial quantities of the polymer are employed to maintain a desirably high viscosity. Moreover, in many cases, particularly in a highly porous zone of relatively low porosity, substantial quantities of the drive fluid are recovered with the hydrocarbon.
As an alternative method for controlling the mobility of a drive fluid and/or water production in an oil-producing well, it has been proposed to plug some of the more porous formations by introducing a water-soluble acrylamide-carboxylic acid copolymer into the formation (see, e.g., U.S. Pat. No. 3,087,543). The polymer is forced into the formation and reduces the permeability of the formation to water without substantially decreasing the permeability of the same formation to hydrocarbon. Unfortunately, any beneficial effects only last for a relatively short time because of the inherent water-solubility of the polymer.
An improved method for controlling the flow of a fluid through a subterranean formation consists of injecting a cross-linked, water-insoluble gel of a water-soluble polymer into the formation (U.S. Pat. No. 3,921,733). Similarly, it has also been proposed to add discrete, spheroidal microgels of a water-swellable or water-swollen cross-linked polymer to the formation to control the mobility of fluids (see, e.g., U.S. Pat. Nos. 4,182,417 and 4,291,069). These cross-linked polymers have been shown to be effective in modifying the permeability of the formation to improve the efficiency of the recovery operation, thereby producing hydrocarbon which contains lesser amounts of water or other drive fluid. Unfortunately, due to their high viscosities, it is often difficult to pump the cross-linked polymer into the formation.
Alternatively, it has also become a practice to modify the control fluids in a formation by introducing a water-soluble polymer into the formation and cross-linking the polymer in situ using a metallic (e.g., sodium dichromate/sodium bisulfite) or an organometallic (e.g., aluminum citrate) cross-linking agent to form a water-soluble gel(see, e.g., U.S. Pat. Nos. 3,780,806; 3,785,437; 3,809,160 and 3,701,384). Unfortunately, the cross-linking of water-soluble polymers such as polyacrylamides and hydrolyzed polyacrylamides using a metallic or organometallic cross-linking agent can be difficult to control. Moreover, other materials present in the formation such as surfactants, soluble anions or dissolved gases can interfere with the cross-linking of the polymer.
In view of the deficiencies of the prior art methods, it remains highly desirable to provide an improved method for modifying the permeability of subterranean structures and hence, the flow of fluids therethrough.